Address restriction is not really much use apart from those customers physically tied to a desk… (and even some of those will be on a connection with dynamic IP)
I've been putting this off for a couple of years now because I thought LicenseChest was on the horizon, but I need to try and find some sort of workaround. The one I'm leaning towards is registering a whole new domain for each customer, which seems crazy wasteful, but cheaper than separate servers!